A. Field of invention
The present invention relates generally to devices for pumping fluids and more particularly to a new and improved pump for dispensing controlled amounts of chemically aggressive or sensitive fluids.
B. Description of Related art
Dispensing controlled amounts of fluids requires a means for precisely controlling the flow through the dispensing device. In particular, it is important to insure that such a dispensing system does not leak or weep when the pump is not in active operation. As exemplified by U.S. Pat. No. 4,832,582 to Buffett, it is known to provide a fluid dispensing pump having one way inlet and outlet valves separated by a chamber of variable volume. One wall of the Buffett pump consists of an oscillating diaphragm which causes the chamber volume to alternately increase and decrease. While the Buffett pump provides a flow of fluid at a relatively constant rate, the integral valves of the Buffett pump only impede flow through the valve by the resilience of the one way duck bill valves at the inlet and outlet ports which provide only slight resistance to opening. Since the factors impeding flow in the Buffett pump would also directly and adversely affect the pump's efficiency, it is to be expected that these factors would be minimized to increase efficiency and conversely decrease resistance to flow. Accordingly, a relatively small positive pressure differential across the Buffett pump would be expected to cause flow to proceed through the pump independent of the operation of the pump. For this reason, conventional pumps such as Buffett are frequently used in applications in which a positive pressure drop across the pump is not anticipated. An example of the limitations of use of such pumps is shown by the placement of the fluid reservoir below the Buffet pump. Such pumps could not be utilized in those applications wherein the fluid reservoir is most conveniently located above the pump as this would create a pressure drop across the pump which could be sufficient to cause the pump to leak when not in operation.
Some other type pumps such as vane or screw types as well may, when new, provide a measure of integral means of flow control by virtue of either positive blockage or impedance of flow; however, as wear occurs at the junction of the vanes or blades and the housing and other parts that move against or past one another, a tight seal is difficult and leakage may commence even in the conventional pumps that are designed to block flow by integral means. Wear of moving parts is particularly important in devices designed for use with chemically sensitive or aggressive fluids because the acceptable materials are limited and are not selected on the basis of wear resistance characteristics. In other systems, flow through the system is controlled by means of an external shut off valve at either the inlet or outlet of the system. In such systems, although the pump mechanism would otherwise leak when not activated, flow is nevertheless blocked by a shut off valve operated in coordination with the pump. A system using both a pump and a separate shut off valve is cumbersome, particularly in applications requiring controlled dispensing of minute amounts or multiple dispensers in a small area. The separate shut off valve may represent an inefficient use of material as a result of the duplication of the flow control function. In addition, the coordination of operation between the valve and pump requires a common control or other means of ensuring simultaneous operation, and the breakdown or other failure of the operation control would be expected to cause the system to experience potentially damaging stress.